This invention relates to a pressure gas engine having improved speed control means for throttling very precisely a gas valve when speeds less than maximum speed are desired and in which the control means comprises a bellows anchored at one end with the other end attached to a movable valve and with both sides of the bellows exposed to the internal gas pressure in the embodiment so as to be substantially in equilibrium together with a speed responsive device for controlling the flow of bleed gas from one side of the bellows for controlling precisely the speed at any particular setting.